Macro-Tech 3600VZ
Power Amplifier
Page 28
Reference Manual
The input signal at the phone jack passes directly into
the balanced gain stage (U104-C,D). Use of a
PIP mod-
ule for input signal causes the input signal to pass
through the
PIP and then to the balanced gain stage.
The balanced gain stage (U104-C,D) causes balanced-
to-single-ended conversion to take place using a differ-
ence amplifier. From there, gain can be controlled with
a potentiometer. The error amp (U104-A) amplifies the
difference between the output signal and the input sig-
nal from the gain pot, and drives the voltage
translator
stage.
The voltage
translator stage channels the signal to the
Last Voltage Amplifiers (LVAs), depending on the sig-
nal polarity, from the error amp U104-A. The +LVA
(Q105,Q125) and the –LVA (Q110,Q126), with their
push-pull effect through the bias servo Q318, drive the
fully complementary output stage.
The bias servo Q318 is thermally coupled to the heat
sink, and sets the quiescent bias current in the output
stage to lower the distortion in the crossover region of
the output signal. D301, D302, D303, and D304 are
used to remove the charge on the unused portion of the
output stage, depending on the polarity of the output
signal.
With the voltage swing provided by the LVAs, the signal
then gains current amplification through the Darlington
emitter-follower output stage.
The bridge-balanced circuit (U104-B) receives a signal
from the output of the amplifier, and differences it with
the signal at the Vcc supply. The bridge-balanced cir-
cuit then develops a voltage to drive the bridge-bal-
anced output stage. This results in the Vcc supply
having exactly one
half of the output voltage added to
their quiescent voltage. D309, D310, D311 and a trim-
mer resistor set the quiescent current point for the
bridge-balanced output stage.
The protection mechanisms that affect the signal path
are implemented to protect the amplifier under real-
world conditions. These conditions are high instanta-
neous current, excessive temperature, and operation of
the output devices outside safe conditions.
Q107 and Q108 act as a conventional current limiter,
sensing current in the output stage. The allowable cur-
rent level is also adjusted as a function of voltage. When
current at any one instant exceeds the design criteria,
the limiters remove the drive from the LVAs, thus limiting
current in the output stage to a safe level.
To further protect the output stages, a specially devel-
oped
ODEP (Output Device Emulation Protection) cir-
cuit is used. It produces an analog output proportional
to the always-changing
safe operating area of the out-
put transistors. This output controls the translator stage
by removing any drive that exceeds the
safe operating
area of the output devices.
Thermal sensor S100 gives the
ODEP circuits vital infor-
mation on the operating temperature of the heatsink on
which the output devices are mounted.
Should the amplifier fail in such a way that would cause
DC across the output lead, the DC protection circuit
senses this on the negative feedback loop and shuts
down the power supply until the DC is removed.
5.3.2 Bridge-Mono Operation
By setting the back panel Stereo/Mono switch to Bridge-
Mono, the user can convert the
Macro-Tech into a
Bridge-Mono amplifier. With a signal applied to the
Channel 1 input and the load between the red binding
posts on the back panel, a double
voltage output oc-
curs.
The Channel 1 output feeds the Channel 2 error amp
U204-A. Since there is a net inversion, Channel 2 output
is out of polarity with Channel 1. This produces twice as
much voltage across the load. Each of the channel’s
protection mechanisms work independently if a fault oc-
curs.
5.3.3 Parallel-Mono Operation
With the Stereo/Mono switch set to Parallel-Mono, the
output of Channel 2 is paralleled with that of Channel 1.
A suitable high-current-handling jumper must be con-
nected across the red binding posts to gain the ben-
efits of this mode of operation.
The signal path for Channel 1 is the same as previously
discussed, except that Channel 1 also drives the out-
put stage of Channel 2. The balanced input, error amp,
translators and LVAs of Channel 2 are disconnected and
no longer control the Channel 2 output stage. The Chan-
nel 2 output stage and protection mechanisms are also
coupled through S1 and function as one.
In Parallel-Mono mode, twice the current of one channel
alone can be obtained. Since the
ODEP circuit of Chan-
nel 2 is coupled through S1, this gives added protec-
tion if a fault occurs in the Channel 2 output stage. The
ODEP circuit of Channel 2 will limit the output of both
output stages by removing the drive from the Channel 1
translator stages.
